The present invention relates to a bent sub assembly for use in directional drilling.
Bent sub assemblies are used to change the direction in which a drilling bit cuts the borehole. The bent sub assembly has a transverse bend at a point along its length. The assembly is commonly positioned between the power section of a downhole motor and a bearing assembly to which the bit is connected. At its upper end, the bent sub assembly is coaxial with the longitudinal axis of the power section and the drill string, but beneath the bend the longitudinal axis of the lower end of the bent sub assembly and the bearing assembly attached thereto is offset at a slight angle (typically up to 3xc2x0).
It is known to those skilled in the art that it is desirable to locate the bend of the bent sub assembly as close to the bit as possible. Every inch counts. The distance of the bend from the bit will affect the xe2x80x9caggressivenessxe2x80x9d and control of the drilling assembly. If the bend is closer to the bit, the drilling assembly will xe2x80x9cbuild anglexe2x80x9d or change direction more quickly than it will if the bend is further from the bit.
It is the purpose of the present invention to provide a bent sub assembly having an effective or resultant bend point which is close to the lower end of the assembly, and thus to the bit.
In accordance with the invention an adjustable, tubular bent sub assembly is provided having two bends at spaced points along its length, so that the assembly has a resultant or effective bend point close to its lower end. The bent sub assembly has longitudinal axes of symmetry at its upper and lower ends which are angularly offset. The assembly further has means for adjusting the extent of the angular offset.
More particularly, in a preferred embodiment, the tubular bent sub assembly comprises:
an internal mandrel which is threadably connected at its upper and lower ends with an external adaptor and an external offset sub, respectively, the mandrel further being disengagably locked by splines or the like with an external adjusting ring positioned between the adaptor and the offset sub, the lower end of the adjusting ring being connected with the upper end of the offset sub by axially disengagable clutch means, so that the assembled components form a solid unit for transmitting rotation from a power unit or downhole motor assembly to a drill bit assembly;
the offset sub is positioned at the base of the assembly and has a transverse bend at a point along its length, thereby defining upper and lower sub sections above and below the bend, each sub section having a longitudinal axis of symmetry, the axis of the upper sub section being perpendicular to the plane of the upper end face of the offset sub, the offset sub being symmetrically internally threaded at its upper end for connection with the mandrel (that is, the threads are coaxial with the upper sub section);
the mandrel is externally threaded at each end and has a transverse bend at a point along its length, close to or at the interface of the offset sub and the adjusting ring, the bend therefore defining upper and lower mandrel sections above and below the bend, each mandrel section having a longitudinal axis of symmetry, the axis of the upper mandrel section being coaxial with the adaptor and the axis of the lower mandrel section being coaxial with the upper sub section, the mandrel having external splines extending longitudinally thereof above its bend point;
the tubular adaptor is straight and has a longitudinal axis which is coaxial with the axis of the mandrel upper sectionxe2x80x94the adaptor is internally threaded at its lower end for connection with the mandrel and is threaded at its upper end for connection with the stator of the power section;
the tubular adjusting ring is straight and has a canted bottom end face that butts up flush to the upper end face of the offset sub. The adjusting ring further has internal, longitudinally extending splines for engaging the mandrel splines to lock the ring and mandrel together for rotation while allowing the ring to be moved axially along the mandrel when the ring is not restrained at its upper endxe2x80x94the ring is adapted to turn with the mandrel about the lower axis of the mandrel, which is coaxial with the axis of the upper section of the offset sub. As a result, the lower end face of the ring remains parallel with the upper end face of the offset sub, so as to butt flush therewith, and the upper axis of the mandrel changes orientation relative to the lower axis of the offset sub when rotated.
By the combination of bending the mandrel at the interface of the ring and the offset sub, bending the offset sub below that interface and canting the lower end face of the ring and the upper end face of the offset sub, one can locate the effective bend point of the bent sub assembly beneath the ring (that is, within the upper sub section of the offset sub). In addition, by locating the second bend in the offset sub, this has made it possible to cut the top thread of the offset sub concentrically, instead of angling it as has been common with prior art bent sub assemblies. As a result, the section modulus remains constant through the cross-section of the upper end of the offset sub, thereby avoiding weakening it as has been the case in the prior art.
Broadly stated, there is provided a bent sub assembly for directional drilling, comprising an external tubular adaptor having an internally threaded lower end; an internal tubular mandrel having externally threaded ends and being transversely bent at a point along its length to define upper and lower mandrel sections having angularly offset axes of symmetry, the upper mandrel section being received in the adaptor lower end and being threadably connected therewith; an external tubular offset sub transversely bent at a point along its length to define upper and lower sub sections having angularly offset axes of symmetry, the upper sub section being internally threaded at its upper end and coaxially receiving the lower mandrel section and being threadably connected therewith, the offset sub having an upper end face perpendicular to the axis of symmetry of the upper sub section; an external tubular adjusting ring having an axis of symmetry and being positioned between and abutting the adaptor and the offset sub, the ring being slidably splined with the mandrel, the ring having a lower end face canted at an angle to the axis of symmetry of the ring; the offset sub upper end face and the ring lower end face being disposed to butt flush against each other when assembled; the combination, of bending the internal mandrel, bending the offset sub and canting the lower end face of the ring, resulting in locating the effective bend point of the bent sub assembly beneath the ring.